In recent years, the mining conditions of oil wells and gas wells (hereinafter, oil and gas wells are collectively referred to simply as “oil wells”) of crude oil, natural gas, and the like have become severer. Concerning the mining environments, as the mining depth increases, the atmosphere contains CO2, H2S, Cl−, and the like, and the mined crude oil and natural gas also contain large amounts of H2S. Therefore, the requirement on performance of a line pipe for transmitting the crude oil and natural gas has also become more rigid, and a demand for steel pipes for line pipe having sulfide resistance has increased.
The standards of National Association of Corrosion Engineers (NACE) specify the highest hardness of steel for steels used in H2S environments from the viewpoint of sulfide stress cracking resistance (hereinafter, referred also to as “SSC resistance”), which is 250 HV10 or lower for carbon steels. Therefore, for steels that are required to have sulfide resistance, the improvement in technique for restraining hardness has become an important problem. The “HV10” means a “hardness symbol” in the case where Vickers hardness test is conducted with the test force being 98.07 N (10 kgf).
In the case where a high-strength seamless steel pipe for line pipe is produced, unlike the production process of UO steel pipes in which controlled rolling is performed, it is a common practice to perform quenching followed by tempering, in order to assure strength. For a low-carbon steel such as a steel for line pipe, by the ordinary quenching and tempering treatment, martensite is not formed, and a micro-structure consists mainly of bainite. Because of great dependence on cooling rate, the micro-structure sometimes differs between the surface and the interior of steel pipe. Therefore, as compared with the interior in which the cooling rate is low, the surface, on which the cooling rate is high, tends to have a high hardness. As the result, the highest hardness on the surface becomes high with respect to the strength of steel. This tendency becomes remarkable as the strength increases and the wall thickness increases because of the increase in amount of added alloying elements.
Patent Document 1 discloses a seamless steel pipe for line pipe and method for producing the same having a high strength of X80 grade (a yield strength of at least 551 MPa) and having improved toughness and corrosion resistance just by heat treatment in the form of quenching and tempering, in the case of a thick-walled seamless steel pipe having a thickness of at least 30 mm. Also, Patent Document 2 discloses high-strength, high-toughness seamless steel pipe for line pipe having a strength of X80 grade.